
FRICTION
117
9.4 Increasing and Reducing
Friction
As you have seen in the previous section,
friction is desirable in some situations.
Have you ever thought why the sole
of your shoe is grooved [Fig. 9.11 (a)]? It
is done to provide the shoes better grip
on the floor, so that you can move safely.
Similarly, the treaded tyres of cars,trucks
and bulldozers provide better grip with
the ground.
In some situations, however, friction
is undesirable and we would want to
minimise it.
Why do you sprinkle fine powder on
the carrom board (Fig. 9.12)? You might
have noticed that when a few drops of
oil are poured on the hinges of a door,
the door moves smoothly. A bicycle and
a motor mechanic uses grease between
the moving parts of these machines. In
all the above cases, we want to reduce
Fig. 9.12 : Powder is sprinkled on the carrom
board to reduce friction
friction in order to increase efficiency.
When oil, grease or graphite is applied
between the moving part of a machine,
a thin layer is formed there and moving
surfaces do not directly rub against each
other (Fig. 9.13). Interlocking of
irregularities is avoided to a great extent.
Movement becomes smooth. The
substances which reduce friction are
called lubricants. In some machines, it
may not be advisable to use oil as
lubricant. An air cushion between the
moving parts is used to reduce friction.
We deliberately increase friction by
using brake pads in the brake system
of bicycles and automobiles. When you
are riding a bicycle, the brake pads do
not touch the wheels. But when you
press the brake lever, these pads arrest
the motion of the rim due to friction.
The wheel stops moving. Y
ou might have
seen that kabaddi players rub their
hands with soil for a better grip of their
opponents. Gymnasts apply some coarse
substance on their hands to increase
friction for better grip.
(a) (b)
Fig. 9.11 : (a) Soles of shoes and (b) tyres are
treaded to increase friction
Fig. 9.13 : Action of lubricant
Surface 1
Surface 2
Lubricant